IB Biology — Gas exchange

Key Idea: Every cell needs oxygen for respiration and must get rid of carbon dioxide. The job of this topic is to move those gases between the body and the air, and it always comes down to one process: diffusion down a concentration gradient, with no energy needed. Six linked questions run through it. What makes a good gas-exchange surface (2.6.1)? How is the mammalian lung built to do this, and what is surfactant for (2.6.2)? How do we actually move the air in and out — the mechanics of breathing (2.6.3)? How do we measure lung volumes off a spirometer trace (2.6.4)? What goes wrong in emphysema (2.6.5)? And how does a leaf exchange gases (2.6.6)? Three ideas tie it together: a good surface is large, thin, moist and permeable; gases move down a gradient that must be kept steep; and structure follows function — every adaptation exists to make diffusion fast. Gas exchange is a regular across all papers — Paper 1A (a quick MCQ on a surface, cell type or lung volume), Paper 1B / Paper 3 (read a spirometer or pressure trace, or trace the CO₂ path on a leaf image) and Paper 2 (explain how the gradient is maintained, how ventilation works, or the changes seen in emphysema).

🫁 Gas-exchange surfaces

A gas-exchange surface is the thin boundary where gases pass between the body and the air (or water) outside. In the mammalian lung that surface is the wall of the tiny air sacs called alveoli. Gases cross it by diffusion — they move on their own from where they are more concentrated to where they are less concentrated, so no energy is used. Everything about a good surface is designed to make that passive diffusion as fast as possible. The same four features appear in every animal, from a fish gill to a human lung: the surface is large, thin (a short diffusion distance), moist and permeable — plus a good blood supply that keeps the concentration gradient steep.

Diffusion only continues while there is a concentration gradient. If the oxygen in the alveolus and in the blood became equal, net diffusion would stop. So the body refreshes both sides: ventilation (breathing) keeps the alveolar air rich (O₂ high, CO₂ low), and blood flow keeps the capillary blood the opposite (O₂ low, CO₂ high). Together they maintain a steep gradient, so O₂ diffuses air → blood and CO₂ diffuses blood → air, and diffusion never stops.

Think 'big, thin, wet — and never let the gap close.' A big, thin, wet surface makes each diffusion fast; a constant supply of fresh air and fresh blood keeps the gradient steep.

💧 Alveoli & the mammalian lung

The lung is filled with millions of tiny alveoli. Splitting the lung into many small sacs packs an enormous surface area into a small chest — and surface area is exactly what controls how fast gases can diffuse. Zoom in on one alveolus and three cell types run it. Type I pneumocytes are thin, flat cells that form most of the wall — they give the short diffusion distance and are the gas-exchange surface itself. Type II pneumocytes secrete surfactant. A phagocyte patrols inside and engulfs any pathogens, dust or debris that are breathed in. Surfactant is the star of the topic. It lowers the surface tension of the watery film lining each alveolus, which does two things: it stops the alveoli collapsing (or sticking shut) as air leaves on exhalation, and it makes the lungs easier to inflate on the next breath.

Type I = thin = exchange (one cell thick). Type II = surfactant (think 'II' = two jobs: stops collapse AND eases inflation). The phagocyte is the cleaner. Surfactant always means reduces surface tension → stops collapse — never 'carries oxygen'.

🌬️ Ventilation mechanics

Ventilation is simply breathing — moving air into and out of the lungs to keep the gas-exchange surface supplied with fresh air. The lungs have no muscle of their own; two sets of muscles change the size of the chest (thorax): the diaphragm (a sheet below the lungs) and the intercostal muscles (between the ribs). The whole of breathing is one chain of cause and effect: muscles → volume → pressure → air flow. Inhalation: the diaphragm and external intercostals contract — the diaphragm flattens and the ribs swing up and out — so the volume increases, the pressure falls below atmospheric, and air flows in. Exhalation (at rest): the muscles simply relax (it is passive), the volume decreases, the pressure rises above atmospheric, and air flows out. Volume and pressure always change in opposite directions, and air always moves from high to low pressure.

Big chest, low pressure, air in. Small chest, high pressure, air out. Make the box bigger and the pressure drops, so air rushes in; make it smaller and the pressure rises, so air is pushed out. The most common lost mark is naming the muscles but never mentioning the volume change — volume is the link that changes the pressure.

📈 Measuring lung volumes

Your lungs never fill and empty completely. At rest you move only a small amount of air, but you can breathe in much more, and force out much more, if you need to. A spirometer measures these volumes and records them as a trace (lung volume against time). The key skill is knowing what each volume means and where to measure it. Tidal volume (TV) is one normal breath — the height of one small resting wave. Vital capacity (VC) is the largest volume moved in one breath — from the top of the deepest breath in to the bottom of the fullest breath out (VC = IRV + TV + ERV). The residual volume (RV) always stays in the lungs, so it never appears on the trace (and total lung capacity = VC + RV). The ventilation rate is the number of complete waves in one minute. Two extra details get tested. One-way valves keep inhaled and exhaled air on separate tubes; soda lime absorbs the exhaled CO₂. Because O₂ is used up in respiration and the exhaled CO₂ is absorbed (not replaced), the total gas in the closed circuit falls, so the resting baseline drifts downward. During exercise the waves get taller and closer together, raising the air inhaled per minute.

Vital capacity is NOT total lung capacity — the residual volume is always left behind, so total lung capacity = vital capacity + residual volume. The downward baseline drift needs both causes: O₂ used up in respiration AND exhaled CO₂ absorbed by the soda lime (so it is not replaced).

🚩 Emphysema & lung disease

Emphysema is a lung disease in which the thin walls between alveoli are destroyed, usually after long-term irritation from cigarette smoke (and air pollution). The damage follows a clear chain of cause and effect. The walls break down, so many small sacs merge into fewer, larger air spaces. This greatly reduces the surface area for gas exchange, so oxygen diffuses into the blood more slowly. The lung also loses its elastic recoil, so stale air is trapped and breathing out becomes hard work. The result is that oxygen reaches the blood too slowly — the person becomes breathless and tires quickly, especially during exercise, when they cannot raise their oxygen uptake enough to meet the higher demand. Because the main cause is smoking, the public-health changes most likely to reduce emphysema target it: anti-smoking laws, stop-smoking support and cutting air pollution.

Think 'fewer, bigger, slower' — emphysema makes fewer, bigger air sacs, so gas exchange becomes slower. For an Explain [4] name both damage routes: less surface area AND lost elastic recoil. For 'how to reduce incidence', target the cause — reduce smoking and air pollution.

🍃 Gas exchange in leaves

A leaf is a thin, flat organ built for gas exchange and photosynthesis. Being thin gives a short diffusion distance; being flat and wide gives a large surface area — both speed gas exchange, exactly as in the lung. Reading from the top down: a waxy cuticle (transparent, waterproof) and upper epidermis (clear, no chloroplasts) let light through; the palisade mesophyll (tall cells packed with chloroplasts) does most of the photosynthesis; the spongy mesophyll (loose cells with large air spaces) lets gases diffuse to and from every cell; and the lower epidermis holds the stomata — pores each flanked by two guard cells that open and close them. The classic exam task is to trace the CO₂ path from air to chloroplast: in through an open stoma → through the spongy-mesophyll air spaces → across the cell wall and membrane → through the cytoplasm → into a chloroplast. O₂ and water vapour leave by the reverse route, all by diffusion.

Stoma → Spaces → Surface → Cytoplasm → Chloroplast. For a 'trace the path' question the examiner wants the steps in order — naming 'diffusion' alone scores nothing. Stomata and guard cells are mostly on the lower (under) surface of the leaf.

✍️ Worked examples

✅ Quick self-check

Tap each card to check yourself.

Key Idea: Gas exchange moves oxygen in and carbon dioxide out by diffusion down a concentration gradient, with no energy needed. A good gas-exchange surface is large, thin (short diffusion distance), moist and permeable, and a good blood supply plus ventilation keep the gradient steep so diffusion never stops. In the mammalian lung the surface is the alveolus: type I pneumocytes form the thin lining, type II pneumocytes secrete surfactant (which lowers surface tension to stop the alveoli collapsing), and phagocytes keep it clean. Air is moved by ventilation — the diaphragm and external intercostals change the volume of the thorax, which changes the pressure (volume ↑ → pressure ↓ → air in; volume ↓ → pressure ↑ → air out), with resting exhalation passive. A spirometer records lung volumes as a trace: tidal volume (one wave), vital capacity (deepest in to fullest out = IRV + TV + ERV) and residual volume (never on the trace); one-way valves separate the airstreams and soda lime absorbs CO₂, which with O₂ being used up makes the baseline drift down. In emphysema, alveolar walls are destroyed → fewer, larger sacs → less surface area and lost elastic recoil → oxygen uptake too slow → breathlessness (main cause: smoking). In a leaf, a thin flat structure exchanges gases through stomata (controlled by guard cells); the palisade mesophyll does most photosynthesis and the spongy mesophyll's air spaces carry gases, with CO₂ following stoma → air spaces → cell wall/membrane → chloroplast. Across lung and leaf alike, the same idea wins: make diffusion fast and keep the gradient steep — structure follows function.